The present invention relates to a system for controlling the traction of wheels of a four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle.
In a four-wheel drive, vehicle, slipping, will occur either of front wheels as a set or rear wheels as a set or all of the four wheels. The slipping of the sets of front wheels or rear wheels can be prevented by locking the central differential. However, since the differential operation is restrained, cornering ability deteriorates. Further, when the central differential is locked, slipping of the wheels can not be detected, so that it is impossible to provide automatic control to release the locking of the differential. Accordingly, the differential is manually operated to release the lock at driving on a snowy road having a low friction coefficient .mu..
Recently, a torque split control system has been provided for controlling the distribution ratio of torque of the engine to the front and rear wheels, when the front wheels or rear wheels slip. For example, if the front wheels slip, the torque split control system operates to increase the torque applied to the rear wheels, so that slipping of the front wheels is prevented. Thus, the vehicle can be driven around a corner with the differential operation.
However, although two-wheel slip can be prevented by controlling the torque distribution ratio, slipping of all of the four wheels can not be prevented by the torque split control. In order to restrain the slipping of the four wheels, for example, the power of the engine is reduced to decrease the traction.
Japanese Patent Application Laid-Open 62-87630 discloses a traction control system. When drive wheels spin, the power of the engine is reduced to decrease the traction, so that spinning of the wheels can be prevented. When the drive wheels spin at a low vehicle speed, reduction of the power of the engine is not allowed so as to ensure starting characteristic.
However, in such a system, since a throttle valve is frequently controlled, the power of the engine becomes unstable. At starting the vehicle, the traction control is not performed, so that spinning of the wheels at starting can not be restrained. Accordingly, it is difficult to prevent the four wheels from spinning only by traction control.